Bluetooth: CCID: Make the CCID API public

Move the CCID header to the include directory
to make it public.

This also compiles the ccid.c file whenver CONFIG_BT_CONN
is enabled, rather than having an additional Kconfig for it,
since the API is now public and may be used by other than
our internal services.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>